Longitudinal and Latitudinal Split-Gate Field-Effect Transistors for NAND and NOR Logic Circuit Applications

Structural engineering of split-gate FET (SG-FET) is carried out to achieve two different homogeneous MoS2-based switching devices, whose SG gap direction are longitudinal (AND-FET) and latitudinal (OR-FET) to the channel direction.
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With eager research to the advanced electronic devices, the two-dimensional van der Waals (2D vdW) materials, which have atomically thin bodies and dangling-bond free surfaces properties, have attracted significant research interest for the development of future electronics and their applications. Among these, molybdenum disulfide (MoS2) is eligible for the field-effect transistor (FET) applications with a high electron mobility and a high ON/OFF drain current ratio, resulting from the presence of bandgap unlike the bandgap-less graphene.

The main approach to the FET device is to realize the low (ON) and high (OFF) resistance states in semiconductor channel depending on the gate-field effect as a switching device. Based on this conventional device configuration, at least two FETs should be required to design the logic gate applications for diverse computation. For example, in the pull-down network (PDN) of inverter circuit applications, the series connection of two n-type FETs works as a NAND logic gate; conversely, the parallel connection of two n-type FETs works as a NOR logic gate. 

In previous studies, dual-gate FETs and split-gate FETs (SG-FETs) were reported to demonstrate these conventional logic gates by using a single active channel. These configurations were enabled by locally controlled active channel based on the multiple gate electrodes, and these application methods could prove that 2D vdW materials have high functionality and exhibit ease of integration in advanced electronic devices. 

In our work published on npj 2D materials and applications, we demonstrated single MoS2 active channel-based AND and OR switching devices, which were realized as different SG-FET configurations, beyond the conventional ON/OFF switching device. As simple structural engineering of the SG-FET, the gap direction of the SG electrode was defined for the longitudinal or latitudinal direction to the channel length direction (Figure 1). Owing to its atomically thin MoS2 layer, each SG electrodes can control the doping profile of MoS2 active channel with different divided areas. In other words, as their gap directions were perpendicular to each other, different switching characteristics, i.e., “AND” (AND-FET, longitudinal SG-FET) or “OR” (OR-FET, latitudinal SG-FET), were realized. Furthermore, the NAND and NOR logic applications are demonstrated by designing the inverter circuit applications of them. These approaches are simply motivated by the structural difference of SG-FETs and realize the multi-functionality and high integration of 2D vdW materials.

 

Beyond the small-footprint logic gate applications, the simple innovation of structural engineering in split-gate structure provides the demonstration of advanced logic applications such as multi-value logic and reconfigurable devices. For example, the multi-value logic could be demonstrated by the asymmetric schematic of the OR-FET, and both top and bottom split-gates structure could make reconfigurable characteristics towards the neuromorphic applications.

For further information, please read our published article "Longitudinal and Latitudinal Split-Gate Field-Effect Transistors for NAND and NOR Logic Circuit Applications" in npj 2D Materials and Applications. 

https://www.nature.com/articles/s41699-022-00320-w

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